1. Field of the Invention
The present invention relates to an automatic transmission and more specifically to an accumulator/friction element arrangement for such a transmission which improves the shift characteristics of the same.
2. Description of the Prior Art
JP-A-61-165054 discloses an automatic transmission arrangement wherein two friction elements (a high clutch and a band brake) are arranged to be controlled by a single shift valve. That is to say, with this arrangement when the shift valve assumes an upshift position the clutch is supplied with hydraulic fluid in a manner to induce the same to engage while the release chamber of the band brake servo is pressurized in a manner wherein it is conditioned to assume a released position.
An orifice is arranged in the conduiting interconnecting the shift valve and the above mentioned friction elements in a manner to restrict the communiction therebetween. An accumulator is also connected with the conduiting arrangement in a manner to communicate with the same at a location between the friction elements and the shift valve. This accumulator is arranged to maintain the pressure in the conduiting at an essentially constant level during the charge stroke thereof and thus reduce shift shock during an upshift.
However, this arrangement has suffered from the drawback that when the throttle opening is relatively large the band brake response is not sufficiently quick during upshifts. That is to say, the accumulator is arranged to, during its charging stroke, modify the pressure in a manner which slows the development of the pressure in the band brake servo release chamber and which acts against that supplied to an apply chamber. Accordingly, at the final stage of shift the accumulator modifies the pressure development with the result that the torque which is transmitted by the band brake is insufficiently reduced and the shift feeling is deteriorated.
In connection with the above problem, when the throttle opening is on the high side, the level of the line pressure also increases. Accordingly, the point at which the release of the band brake is achieved is determined by the point at which engagement pressure is neutralized by the release pressure. However, when the line pressure is high, the engagement pressure is also high and the release pressure must be raised to a relatively high level before the brand brake can be induced to assume a released condition. However, when the throttle opening is large the throttle pressure developed in the system is increased with the result that the level of line pressure also increases.
In order to overcome this problem it is possible to set the level of the pressure which is maintained by the accumulator higher than the pressure required to release the band brake and thus ensure that the band brake will be released before the accumulator begins to stroke.
While it is possible using the above mentioned technique possible to overcome the delayed band brake release problem a secondary drawback is induced that the band brake tends to be released excessively quickly and invites engine racing.
That is to say, when the throttle opening (pressure) is small the level of the line pressure which is supplied is also reduced with the result that the pressure at which the band brake releases is also reduced. In addition, if the accumulator is arranged to stroke after the pressure has risen to a relatively high level (and therefore temporarily maintain the pressure essentially at this level), the provision of the accumulator tends to be lost as the release pressure is developed before the accumulator actually begins to produce its pressure modifiying effect. Moreover, the engagement timing of the high clutch is essentially uneffected by the level of the line pressure and is essentially fixed.